Pulsar jackpot reveals globular cluster’s inner structure

Graphic showing locations of millisecond pulsars inside
the globular cluster Terzan 5 in an optical image taken by the
Hubble space telescope. Pulsars represented in blue are
accelerating toward observers on Earth; those in red are
accelerating away. These relative accelerations were derived by
measuring minute changes in the speed of rotation of the
pulsars. Credit: B. Saxton (NRAO/AUI/NSF); GBO/AUI/NSF;
NASA/ESA Hubble, F. Ferraro

The Milky Way is chock full of star clusters. Some contain
just a few tens-to-hundreds of young stars. Others, known as
globular clusters, are among the oldest objects in the
Universe and contain up to a million ancient stars.

Some globular clusters are thought to be
fragments of our galaxy, chiseled off when the Milky Way was in
its infancy. Others may have started life as standalone dwarf
galaxies before being captured by the Milky Way during its
formative years.

Regardless of their origins, many globular clusters reside
either in or behind the dusty regions of our galaxy. For
ground- and space-based optical telescopes, however, this poses
a challenge. Though it is possible to observe the cluster as a
whole, the dust hinders astronomers‘ efforts to study the motions of
individual stars. If astronomers could track the
motions of individual stars, they could see how “lumpy” the
globular cluster is or if it contains something really dense,
like a giant black hole at its center.

Fortunately, radio waves—like those emitted by pulsars—are
unhindered by galactic dust. So rather than tracing the motions
of the stars, astronomers should be able to map the motions of
pulsars instead. But, of course, things are never that simple.
Though globular clusters are brimming with stars, they contain
far fewer pulsars.

“That’s what makes Terzan 5 such an important target of study;
it has an unprecedented abundance of pulsars – a total of 37
detected so far, though only 36 were used in our study,” said
Brian Prager, a Ph.D. candidate at the University of Virginia
in Charlottesville and lead author on a paper appearing in theAstrophysical Journal. “The more pulsars you can
observe, the more complete your dataset and the more details
you can discern about the interior of the cluster.”

The Terzan 5 cluster is about 19,000 light-years from Earth,
just outside the central bulge of our galaxy.

For their research, the astronomers used the National Science
Foundation’s (NSF) Green Bank Telescope (GBT) in West Virginia.
The GBT is an amazingly efficient instrument for pulsar
detection and observation. It has exquisitely sensitive
electronics, some specifically optimized for this task, and a
100-meter dish, the largest of any fully steerable radio
telescope.

Pulsars are neutron stars – the fantastically dense remains of
supernovas—that emit beams of radio waves from their magnetic
poles. As a pulsar rotates, its beams of radio light sweep
across space in a cosmic version of a lighthouse. If the beams
shine in the direction of Earth, astronomers can detect the
exquisitely steady pulses from the star.

As the pulsars in Terzan 5 move in relation to Earth – drawn in
different directions by the varying density of the cluster—the
Doppler effect comes into play. This effect adds a tiny delay
to the timing if the pulsar is moving away from Earth. It also
shaves off the tiniest fraction of a millisecond if the pulsar
is moving toward us.

In the case of Terzan 5, astronomers are particularly
interested in a class of pulsars known as millisecond pulsars. These pulsars rotate
hundreds of times each second with a regularity that rivals the
precision of atomic clocks on Earth.

Video describing how astronomers traced the motions of 36
rapidly rotating pulsars inside Terzan 5 – a massive, ancient
star cluster near the center of the Milky Way — to get a clearer
picture of the cluster’s interior and likely birthplace. Credit:
B. Saxton (NRAO/AUI/NSF); GBO/AUI/NSF; NASA/ESA Hubble

Pulsars achieve these remarkable speeds by siphoning off matter
from a nearby companion star. The infalling matter hits the
edge of the neutron star at an angle, increasing the pulsar‘s rate of spin in much the same way that a
basketball balanced on the tip of a finger can be spun up by
striking its side.

Millisecond pulsars are a particular boon to astronomers
because they make it possible to detect almost infinitesimally
small changes in the timing of the radio pulses.

“Pulsars are amazingly precise cosmic clocks,” said Scott
Ransom, an astronomer with the National Radio Astronomy
Observatory (NRAO) in Charlottesville, Virginia, and coauthor
on the paper. “With the GBT, our team was able to essentially
measure how each of these clocks is falling through space
toward regions of higher mass. Once we have that information,
we can translate it into a very precise map of the density of
the cluster, showing us where the bulk of the ‘stuff’ in the
cluster resides.”

Previously, astronomers thought that Terzan 5 might be either a
warped dwarf galaxy gobbled up by the Milky Way or
a fragment of the galactic bulge. If the cluster were a
captured dwarf galaxy, it might also harbor a central
supermassive black hole, which is one of the hallmarks of all
large galaxies and can be found in many dwarf galaxies as well.

The new GBT data, however, show no obvious signs that a single,
central black hole is lurking in Terzan 5. “However, we can’t
yet say for sure if a smaller, intermediate mass black hole
resides there. The new observations also provide better
evidence that Terzan 5 is a true globular cluster born in the Milky Way rather than the
remains of a dwarf galaxy,” said Ransom.

Future observations using more sophisticated acceleration
models may better constrain the origin of Terzan 5.